The present application generally relates to methods for measuring the amount of one phase in a mixture of phases, more particularly to measuring the amount of water present in crude petroleum oil, and most particularly to low water cut crude petroleum oils.
The following paragraphs contain some discussion, which is illuminated by the innovations disclosed in this application, and any discussion of actual or proposed or possible approaches in this Background section does not imply that those approaches are prior art.
Background: Water Cut Analyses in Oil Processing
The chemical and physical characterization of crude, partially refined, and fully refined petroleum products is a common practice in the petroleum industry. Characterizations such as compositional, electrical, and physical property determinations are used for a variety of purposes. One of their more important uses is when they are done in combination with oil well testing to assist in optimizing oil production from a single or series of oil wells. Another important use is during the transfer of crude petroleum oil, as occurs during the production, transport, refining, and sale of oil. Specifically, it is well known to a person having ordinary skill in the art of petroleum engineering that crude petroleum oil emerging from production wells can contain large amounts of water, ranging from generally about 1% to as high as about 99% water. This value is known as the water cut (“WC”).
When water is pumped to the surface of the Earth along with crude petroleum oil, producers often attempt to physically separate the water from the oil, because the water can corrode pipes and damage down-stream processing equipment. Further, the water has no value relative to the oil and in-fact can become a disposal or environmental problem wherever it is finally removed. Water-oil “separators” or liquid-liquid decanters are thus often used, before the crude petroleum oil is further transported from a well site or tank farm. However, the efficiency of such separators in achieving two pure streams of oil and water is often not 100%, and free water is still frequently entrained in the crude petroleum oil as it enters storage, in the range of about 0.10% to about 5%.
The accurate determination of water content and validation of the amount of water in crude petroleum oil is particularly important during the taxation of crude petroleum oil and the sale of crude petroleum oil, where the owner or seller of the oil does not want to pay taxes on water and the customer does not want to pay the price of oil for water. Such determinations and validations can be conducted on-line and off-line during petroleum processing.
The offline method involves physically sampling the stream and analyzing it in a laboratory setting. In the petroleum industry, the sampling is usually done using a composite sampler which automatically opens a sample valve attached to a pipeline at some frequency to collect an aggregate sample into a sample container. The objective is to collect a sample which is representative of the entire lot of petroleum under consideration. After collection, the composite sample is usually picked up by a person and taken to a laboratory. The composite sample is then “sampled” to prepare aliquots, or sub-divisions of the composite sample, for each of the various characterizations, or analysis methods, to be used.
Three off-line analytical methods are commonly used for determining the water content of crude petroleum oil. These are the centrifuge method, the distillation method, and the titration method. See the American Petroleum Institute (“API”) Manual of Petroleum Measurement Standards, Chapter 10. The distillation and titration methods are relatively accurate, but are plagued by long analysis times and not suitable for use in the field or at the point of sale. The centrifuge method is quicker, but almost always under-reports the amount of water present. The American Society for Testing of Materials has reported the standard analytical errors for water content measurements using the three methods. The repeatability errors are 0.11% for the distillation method (see ASTM D4006), 0.15% for the titration method (see ASTM D4377), and 0.28% for the centrifuge method (see ASTM D4007).
Note that composite petroleum samplers and the associated analytical methods have other kinds of problems and disadvantages other than, for example, meeting a desired accuracy for a given determination. For example, results for composite samplers are typically only available at the end of a batch or a test, and there is no recourse if something goes wrong with the sampling system during the sampling process. At the end of the sampling and analysis, only a single number is available to consider. Additionally, the exposure of personnel to hazardous liquids associated with processing the samples is undesirable. Thus, the petroleum industry has continued to seek other methods that provide the required accuracy, speed, and safety.
Accordingly, the use of rapid on-line instruments such as densitometers, capacitance probes, radio frequency probes, and microwave analyzers to measure water content of petroleum products is becoming more common. In addition to providing increasingly accurate determinations of water content, real time water content results via on-line methods can provide beneficial operational advantages. Knowledge of when water becomes present in petroleum as it is being produced and the magnitude of the quantity of the water may provide an opportunity to remove the water before it reaches a transport pipeline, storage vessel, or shipping tanker. Additionally, the real time data may show if the water is detected in several short periods of time or if it is present across the entire load of the petroleum. Furthermore, real time analyzers may be used as a comparison to the results provide by composite samplers. Finally, on-line measurements of, for example, physical and electrical properties, via instrumentation reduces the need human involvement in the process of characterizing a multiphase fluid mixture.
Background: Water Cut by the Density and Permittivity Methods
On-line densitometers can be used to ascertain the amount of water in petroleum oil. One on-line density method uses a Coriolis meter. This meter can be installed in the pipeline leaving the well or wells on the way to further processing and storage. Coriolis meters measure the density of a fluid or fluid mixture, and usually its mass flow rate as well, using the Coriolis effect. Then, calculations can be performed to indirectly determine the water percentage. For example, a Coriolis meter can measure the density of a water-oil mixture, ρmixture, and then perform a simple calculation method to determine the individual fractions or percentages of the water phase and oil phase. By knowing or assuming the density of the dry oil, ρdry oil, and the density of the water phase, ρwater phase, then a water weight percentage, Ψwater, can be calculated as follows:Ψwater phase=((ρmixture−ρdry oil)/(ρwater phase−ρdry oil))×100
Note that the above equation can work equally well using the specific gravities of the mixture, dry oil, and water phase, where specific gravity is the ratio of the particular density to the density of water at 4 degrees Celsius.
It should be recognized that the water percentage by density method is subject to uncertainty. First, due to natural variations of, for example, the hydrocarbon composition of crude petroleum oil, the density of the dry oil can vary significantly from the assumed or inputted value required for the simple calculation. Such a value inputted into a densitometer based on a guess or on history of a given oil well. Crude petroleum oils can range from about 800 kilograms per cubic meter (kg/m3) to about 960 kg/m3. Further, the water encountered in oil well production is most often saline. This salinity is subject to variability, ranging from about 0.1% by weight salt to about 28%. This results in a variation in the density of the water phase from about 1020 kg/m3 to about 1200 kg/m3. Again, this value would be inputted into a densitometer based on a previously known laboratory number or on the history of a given well.
Note also that an entrained gas phase, as is sometimes present, can dramatically affect the density of a crude petroleum oil stream as measured by a Coriolis meter, unless a precise correction method is applied for the presence of the gas.
Another technique to determine the water percentage is to use a microwave analyzer, instead of a densitometer, to perform the in-line monitoring of the oil and water mixture.
U.S. Pat. No. 4,862,060 to Scott (the '060 patent), entitled Microwave Apparatus for Measuring Fluid Mixtures and which is hereby incorporated by reference, discloses microwave apparatuses and methods which are most suitable for monitoring water percentages when the water is dispersed in a continuous oil phase.
Note that the change in fluid mixture dielectric properties for a water and oil mixture can be affected by a number of parameters, including not only the percentage of water in oil, but also the individual dielectric constants of the oil phase and the water phase. For example, the dielectric constant of the dry crude petroleum oil itself can vary depending on its density and chemical composition. Note that temperature can affect the density of the oil and the water and thus the dielectric properties of each component and the mixture. However, temperature variations can easily be compensated for by using a temperature probe in-contact with the multiphase fluid being characterized to allow referencing to data sets or curves fit to the data sets for different temperatures.
Thus, both the densitometer method (“WC by density”) and the permittivity method (“WC by permittivity”) are subject to uncertainties. One approach to dealing with the uncertainty is to simultaneously use both methods to characterize a crude petroleum oil stream for water content. This joint use is practiced commercially. An example is the Compact Cyclone Multiphase Meter manufactured by Phase Dynamics, Inc. of Richardson, Tex.
When conducting joint densitometry and permittivity characterizations of a flow stream of mixtures of water and crude or partially refined petroleum oils, exact values of the electrical and physical properties of the pure water and oil phases are not always known. However, in certain situations, each method can supply estimates of some of the required values to assist each other in determining water content in petroleum products.
An example of a such a supply of a physical property estimate is disclosed in U.S. patent application Ser. No. 11/273,613 to Bentley N. Scott entitled Methods for Correcting On-Line Analyzer Measurements of Water Content in Petroleum, and is hereby incorporated by reference, and hereinafter referred to as Scott '3613. Scott '3613 discloses that because a conventional permittivity analyzer is usually shop-calibrated across a range of water contents using a dry oil of a known density, the analyzer will report an erroneous water percentage if the dry oil being measured in the field shifts to a different density than that of the original dry calibration oil. The auto-correction method disclosed in Scott '3613 ameliorates this problem. Scott '3613 teaches that there is 0.03% WC by permittivity error introduced for every 1 kg/m3 shift in actual dry oil density from the dry oil calibration density. It discloses that for WC's less than about 5%, the density of the actual dry oil can be adequately estimated for use in calculations by the permittivity meter by assuming the actual dry oil density is equal to the density of the mixture as measured by the densitometer. This assumption results in a maximum error rate of about 0.23% at about 5% WC. This error rate compares favorably to the off-line analytical method error rates previously detailed.
Background: Crude Petroleum Oil Phase Behavior and Permittivity
Still further uncertainty in conducting characterizations of multiphase fluids such as crude petroleum oil can be caused by both the physical chemistry of each of the fluids and the multiphase fluid mixture itself. In the case of liquid-liquid mixtures undergoing mechanical energy input, the mixture usually contains a dispersed phase and a continuous phase. So, in the example of water and oil, the mixture exists as either a water-in-oil or an oil-in-water dispersion. When such a dispersion changes from aqueous phase continuous to non-aqueous phase continuous, or vice-versa, it is said to “invert the emulsion phase.”
Dispersion of one phase into another becomes more stable under mechanical energy input such as agitation, shaking, shearing, or mixing. These resulting physical properties are known as the rheological properties of the fluids. When the mechanical energy input is reduced or eliminated, coalescing of the dispersed phase can occur, where droplets aggregate into larger and larger volumes. However, these can also be very stable with time depending upon the natural surfactants, densities, temperatures, and salinity of the water. Further, in a substantially static situation (e.g. reduced energy input), heavy phase “settling-out” or stratification can occur under the force of gravity.
Stratification of a lot of crude petroleum oil can lead to sampling difficulties. Processors will often attempt to mix the stratified liquids to allow more uniform sampling. Mixing of large volumes of stratified crude petroleum oil, say 4.1 million barrels contained in an ocean-going supertanker is impractical. Auto-sampling of such stratified volumes is usually done, but with the attending problems as previously described, the least of which is not the fact that samples can get lost, destroyed, spilled, or consumed during repeat testing.
A further complicating phase-state phenomena of liquid-liquid mixtures is that stable or semi-stable suspensions of dispersed-phase droplets can sometimes occur. This is usually referred to as an emulsion, which can be either stable or semi-stable. Certain substances are known as emulsifiers and can increase the stability of an emulsion, meaning that it takes a longer time for the emulsion to separate into two phases under the force of gravity or using other means. In the case of petroleum oils, emulsifiers are naturally present in the crude petroleum oil. For example, very stable emulsions can occur during petroleum processing, as either mixtures of water-in-oil or oil-in-water with the water-in-oil as a stable emulsion possible even up to 90% water.
Another complicating phenomena is that the formation of dispersions and emulsions are sometimes “path-dependent.” Generally, path-dependence exists when the result of a process depends on its past history, i.e. on the entire sequence of operations that preceded a particular point in time, and not just on the current instantaneous conditions. In the case of emulsions, the process of forming the emulsion can be path dependent because the sequence of phase addition, mixing, and energy inputs can affect which phase becomes the dispersed phase and how stable the resulting emulsion is. Thus, if one does not know the history of the multiphase fluid undergoing dispersion or emulsification, one will not always be able to predict the “state” of the dispersion or emulsion, i.e. which phase is continuous and which is dispersed, even if the proportions of the phases are accurately known at a particular point in time.
For permittivity analyzers, whether a dispersion or emulsion is water-continuous or oil-continuous has a significant effect on the analyzer's measurements. In the case of water-continuous dispersions or emulsions, the conductivity path established by the water continuous phase causes a significant change in the measured permittivity relative to the same proportion of phases existing as an oil continuous dispersion or emulsion. Additionally, further variations in the conductivity of the aqueous or water continuous phase caused, for example, by even relatively small changes in salinity, can significantly affect the measured permittivity results. Note that when the non-aqueous or oil phase is continuous, no conductivity path is established (because the droplets are not “connected” to form a continuous conducting circuit) and hence there is no significant effect on the measurements of a permittivity analyzer due to the conductivity of the aqueous phase. Note also that this is only true when the wavelength of the electromagnetic energy is large compared to the emulsion size. When the emulsion size is larger than one eight of a wavelength the voltage difference across the emulsion can be significant and therefore a correction must be made with respect to the salinity (conductivity at the frequency of measurement) of the water.
As a particular example of the complex behavior of liquid-liquid mixtures and the impact of that behavior on electrical characterizations such as permittivity analyses, consider FIG. 1A. It is a generalized phase diagram 100 of a particular crude petroleum oil and a range of aqueous solutions of varying salinity where the fraction of the water phase, XW, is plotted against the frequency, f, as instantaneously read by a microwave permittivity analyzer. Note that although the lines are shown as straight lines, the relationship between XW and f may not be strictly linear. To illustrate aspects of the complex behavior of liquid-liquid mixtures, consider starting with a pure oil phase that is under-going a given amount of mechanical energy input, as is encountered when such a fluid is pumped through a restricting valve and is experiencing a pressure drop. This starting composition, on the path independent, oil-continuous line 101, is represented by point 102. Then, an aqueous saline solution could be added to the oil phase to form a mixture of water-in-oil, represented by points on line 101. The relationship between the permittivity frequency and the aqueous phase fraction is determined by the line 101. On this line, the multiphase fluid exists as an oil continuous phase with drops of dispersed aqueous phase. Then, increasing amounts of saline solution can continue to be added, up along line 101 to point 104. At point 104, the dispersion progresses along path dependent line 105 to point 106. At point 106, the dispersion inverts to an aqueous phase continuous dispersion, with an accompanying discontinuity in measured permittivity, jumping to a particular permittivity curve depending to a large extent on the salinity of the aqueous phase. Aqueous phase can continue to be added along salinity iso-lines in zone 107 to path-independency transition level 108. At path-independency transition level 108, path dependency is no longer present as the dispersion moves into zone 109. The fraction of aqueous phase can be increased to 1.00, with the permittivity being dependent on both the salinity and the fraction of the aqueous phase.
It should be noted that in certain emulsions, zone 107 may not exist at all, and line 105 might transition directly to zone 109.
In an another example of possible path dependency, the mixture may begin as a point located some where in a high water cut, path independent, salinity-controlling, aqueous continuous zone 109. Then, the aqueous fraction could be reduced to path-dependency transition level 110, and further reduced to aqueous fraction 112, along the iso-salinity lines within the high water cut, path dependent, aqueous-continuous zone 111. The iso-salinity lines within zone 111 are shown as dashed lines because they represent salinity levels which may be the same as that in zone 107. Additionally, path-dependency transition level 110 may or may not be equal to path-independency transition level 108.
Following the iso-salinity lines through zone 107, the dispersion would invert at aqueous fraction 112, and as aqueous fraction is further reduced, the relationship follows oil-continuous, path-dependent line 113 to point 104.
It should be noted that in certain emulsions or dispersions, zone 111 may not exist at all, and line 113 might transition directly from zone 109.
Thus, for the particular crude petroleum oil example above as it is mixed in various proportions with a variable salinity aqueous phase, at least three zones of compositional uncertainty can exist for the permittivity of aqueous continuous dispersions, of which at least two such zones can be path-dependent. Additionally, at least three discrete curves can further relate the permittivity of oil-continuous mixtures, of which at least two such curves can be path dependent. In addition, the oil continuous region is dependant upon the frequency of operation as to whether salinity has any affect on the relationship with water percentage as described earlier with respect to the wavelength of the electromagnetic energy.
Such complex physical chemistry leads to numerous uncertainties with regards to permittivity-based composition determinations. For example, referring again to FIG. 1A of this application, frequency 114 can in-fact represent two different mixture compositions, 116 and 118, depending on how such compositions were formed, as previously described. Additionally, a particular aqueous fraction 119 can correspond to either an aqueous phase dispersion of varying salinity contents, points 120, each having a corresponding permittivity frequency (not shown) or an oil-continuous phase dispersion of a particular frequency 122.
It has been found that these compositional and permittivity frequency uncertainties can be reduced by using a number of methods, depending somewhat on which zone or curve the mixture state resides in or on. For example, to address the problems of phase inversion uncertainties in aqueous and non-aqueous multiphase mixtures, U.S. Pat. No. 4,996,490 to Scott (the '490 patent), entitled Microwave Apparatus and Method for Measuring Fluid Mixtures and which is hereby incorporated by reference, discloses microwave apparatuses and methods for accommodating phase inversion events. For the example of oil and water mixtures, the '490 patent discloses that whether a particular mixture exists as an oil-in-water or a water-in-oil dispersion can be determined using differences in the reflected and lost microwave power curves in the two different states of the same mixture. Therefore, the '490 patent disclose microwave apparatuses and methods, including the ability to measure microwave radiation power loss and reflection to detect the state of the dispersion. In further embodiments of that invention, methods are disclosed to compare the measured reflections and losses to reference reflections and losses to determine the state of the mixture as either water-in-oil or oil-in-water, which then allows the proper selection and comparison of reference values relating the measured microwave oscillator frequency to the percentage water. An embodiment of the '490 patent is reproduced from that patent in FIG. 1B, which explained and described in detail later in this Application.
Thus, referring again to FIG. 1A of this application, for water fraction 119, the apparatus and the method of the '490 patent would be able to identify whether the dispersion is in zone 111 or on line 105. When the composition is on line 105, microwave analyzers using the method of the '490 patent are able to accurately determine the aqueous phase fraction.
Thus, solving the problem of accurately ascertaining and validating the amount of each phase in multiphase mixtures is a long felt requiring a more complete and automated solution. More particularly, there is an increasing need for reduction of uncertainty in the characterization of petroleum as the value of petroleum continues to rise.